I'm pulling my rear apart this weekend to check what ratio the gears are (previous owner doesn't know) and I want to replace the gear oil with nice synthetic Mobil 1. What weight oil do I need? Standard 90 weight?

I am running Royal Purple 75W90. I also plan to go to 3.73's and will be using this same gear oil as well. Maybe their 85W140 might go in when I do my gear swap.

Try this
http://www.royalpurple.com/rp-which-oil.html

Do these cars have gaskets or are they the kind where you just put some instant gasket on the cover and then bolt it up?

Well my buddy is a guy who uses nothing but RTV Sealant, but then there is me, who likes gaskets with RTV Sealant for extra comfort. Then there are guys who like just gaskets and hate that RTV stuff unless it is absolutely needed (China walls of the block for the intake manifold).

I think you will be fine either way, don't quote me on that, I am learning myself.

Mobil 1, 75-90 with GM limited slip additive if your car is so equipped, along with a gasket.

JamesC

Thanks for that. How much oil do these need?

About two quarts. You might find the following link informative:

http://www.thirdgen.org/techboard/tr...-10-bolts.html (FAQ About 10-Bolts)

JamesC

I actually read through a lot of that, it's what spurred my desire to rip open the rear diff cover in the first place.

Previous owner claims the car has a Z/28 rear and is limited slip. I have no idea if this is true. I believe the rear end was changed for something with a shorter ratio (the speedo is way off), but I don't know how short.

So I had my rear open today, seems to have a 3.42 gearset and an open diff. I'm pleased with the ratio, a bit peeved that the car has no LSD. No one carried a diff gasket for my car either, just used RTV sealant and it seems fine.

At least you know for sure now. Any plans to swap in an LSD?

I would definitely like a LSD once my engine output warrants the need for such a device. Once I have my TKO swap done I'll be headers, intake, and a cam away from around 300-350 lb-ft so I'll need something soon.

What sort of LSD options are available to me? I've seen stuff about a series 2 and series 3 diff that I don't quite get. I think I have a series 3, but I'm unsure of the axle spline count.

Eaton is a good choice. With 3.42's you have a series III. Your 89 (if indeed that's the year the rear is from) probably has 26 splines, though some 28's did creep in that year.

JamesC

Is there anyway to tell what year the rear is or how many splines it has externally? I doubt I can place any faith in it being a 89. Do I have to pull the axles and count the splines?